1. Field of the Invention
This invention relates to a DC commutatorless motor, and more particularly to a DC commutatorless motor which is useful as a motor for driving a cylinder-drum of a video tape recorder (VTR) or a spindle motor of a floppy disk drive apparatus.
2. Description of the Prior Art
In recent years, DC commutatorless motors are widely used in VTRs and floppy disk apparatuses. When used for driving a cylinder drum in a VTR, such a motor is required to generate a signal having one pulse for every rotation of the rotor coupled to the cylinder drum, so that the rotating position of a rotary magnetic head of the drum can be detected. Hereinafter, such a signal is referred to as "a PG signal". When used as a spindle motor of a driving unit of a floppy disk, such a motor is also required to generate one PG signal (in this case, called an index pulse) for every rotation, thereby defining a write start position of a floppy disk.
FIG. 7 shows a conventional DC commutatorless motor comprising means for generating a PG signal. The motor of FIG. 7 includes a rotor 51 and a stator 52. The rotor 51 comprises a rotor yoke 34, rotor magnets 40 fixed to the rotor yoke 34, and a PG magnet 41 fastened on the periphery of the rotor yoke 34. The stator 52 comprises a stator plate 37, stator windings 35 disposed on the stator plate 37, a stator base 36, a bearing boss 43, and a bearing 39 mounted to the bearing boss 43. A Hall IC 42 is mounted to the stator plate 37 so as to oppose the PG magnet 41, and rotation detecting elements 44 are disposed on the stator plate 37 so as to face the rotor magnets 40. A shaft 38 is attached to the bearing 39. The rotor yoke 34 is fixed to the shaft 38 to rotate thereabout. In the motor having this structure, the Hall IC 42 detects a magnetic flux generated by the PG magnet 41 to produce a PG signal for every rotation of the rotor. The rotation detecting elements 44 detect the rotation of the rotor yoke 34, thereby enabling the control of driving the motor. The rotation detecting elements 44 may be Hall ICs.
In a conventional DC commutatorless motor having a structure as described above, the PG magnet 41 is fixed to the rotor yoke 34 by means of an adhesive agent or the like, thereby increasing the production steps and also the number of components. Moreover, the use of the Hall IC 42 and the magnet 41 for activating the Hall IC causes a considerable rise of the material cost and a further increase of the production steps. Since a usual ferrite magnet cannot generate a magnetic flux sufficient for activating the Hall IC 42, it is necessary to use a rare-earth magnet, which is expensive, as the PG magnet 41 so that a magnetic flux sufficient for activating the Hall IC 42 can be generated, resulting in a further rise of the material cost.